Thz semiconductor laser incorporating a controlled plasmon confinement waveguide

ABSTRACT

A semiconductor laser comprises an active region ( 12 ) which, in response to a pumping energy applied thereto, can produce a stimulated emission of radiation with a central wavelength (λ) in the far infrared region, and a confinement region ( 16, 18, 22 ) suitable for confining the radiation in the active region ( 12 ), and comprising at least one interface ( 16   a,    16   b,    22   a ) between adjacent layers that is capable of supporting surface plasmon modes generated by an interaction of the interface with the radiation. The confinement region ( 16, 18, 22 ) comprises a wave-guide layer ( 16 ) which is delimited on opposite sides by a first interface and by a second interface ( 16   a,    16   b ). The guide layer ( 16 ) is doped in a manner such that the first and second interfaces ( 16   a,    16   b ) are capable of supporting the plasmon modes, respectively, and is of a thickness (d) such as to bring about the accumulation of the plasmon modes in proximity to the first and second interfaces ( 16   a,    16   b ), outside the layer ( 16 ), and substantially a suppression of the plasmon modes, inside the layer.

The present invention relates to a semiconductor laser of the typedescribed in the preamble to Claim 1.

It is generally known that the 1-10 THz frequency region (also definedas far infrared) is difficult to reach with sources based onsemiconductor devices or, more generally, solid-state devices (R. E.Miles et al., Terahertz Sources and Systems, NATO ASI Series, Kluwer2001). In fact, electronic components based on the oscillation of freecharges, such as Gunn diodes or resonant tunnel-effect diodes, can reachfrequencies of about one hundred GHz at most. At the other end of thespectrum, conventional diode lasers operating on optical transitionsfrom the conduction band to the valence band of the semiconductormaterial are typically limited to visible or near/middle infraredfrequencies (>30 THz).

There is, however, very great technological interest in this region ofthe spectrum in view of the many requirements in the fields ofspectroscopy, of wireless communications, and of the production ofimages for medical purposes or security checks. In fact, the particulartransparency or opacity characteristics of various substances withinthis frequency range are very suitable for the examination of biologicaltissues (in a manner similar and complementary to X-rays) or for use insurveillance operations in which it is necessary to examine objects thatare concealed from view by garments or plastics containers. Finally, thetransparency of construction materials and the large bandwidth availablemake these frequencies an optimal choice for intra-buildingcommunications of the future.

In principle, the quantum-cascade lasers (QCLs) that have recently beendeveloped offer the capability to generate electromagnetic radiation inthe far infrared range. These are, in fact, unipolar devices operatingon transitions between sub-bands of states belonging to the sameconduction band, resulting from the quantum confinement of the electronsin a substantially two-dimensional heterostructure (J. Faist et al.,Science 264, 553, 1994). The energy separation between these sub-bands,and hence the frequency of the photons emitted, therefore depends mainlyon the thickness of the semiconductor layers in which the electrons areconfined and not on the electronic structure of the original material.In the current state of the art, QCLs have been produced to cover theentire middle infrared range up to a maximum wavelength of 24 μm (12.5THz) (R. Colombelli et al., Appl. Phys. Lett. 78, 2620, 2001). However,the production of a QCL operating in the THz range has remainedimpracticable up to now for various reasons. In the first place, thereis the need to develop wave-guides of thicknesses (about 10 μm)compatible with the QCL growth system (molecular-beam epitaxy or MBE),which can effectively confine radiation of much longer wavelengths (˜100μm) without increasing optical losses to prohibitive values. In thesecond place, there is the need to design the active region in a mannersuch as to ensure the population inversion that is necessary tocompensate for the cavity losses. This latter need is more complex thanin conventional QCLs owing to the fact that the energies involved becomeless than that of the optical phonon. This completely changes thedynamics of the non-radiative relaxation processes and requires adifferent approach on which to base the creation of the electronicstructure.

In the current state of the art, there are therefore only QC devicesthat are capable of spontaneous emission at the frequencies of interestherein (with powers of the order of tens of pW in the THz range),without any evidence of laser effect or, even less, of gain (M. Rochatet al., Appl. Phys. Lett. 73, 3724, 1998 and J. Ulrich et al., Appl.Phys. Lett. 76, 19, 2000).

The present device, like other semiconductor lasers, is composed of anactive material in which the electromagnetic radiation is generated byvirtue of electron injection. This is introduced into a wave-guide whichis capable of confining the radiation in the particular region of spacewhich is occupied by the active material and which defines the lateraldimensions of the optical cavity that is necessary for the operation ofthe laser. Given the two-dimensional characteristic structure of theactive regions of QCLs, it is necessary to implement a planar wave-guidewhich provides for the confinement of the radiation in the direction inwhich the semiconductor material is grown, leaving the definition of thecavity in the perpendicular directions simply to the processes by whichthe device is produced (lithography, etc.). At visible or near andmiddle infrared frequencies, this wave-guide is generally produced byenclosing the active material between two or more layers of a differentsemiconductor with a lower refractive index. As is known, by virtue ofthe principle of total internal reflection, a wave-guide generallycalled a dielectric wave-guide, with operation similar to opticalfibres, is thus produced. However, this approach cannot be used forfrequencies in the THz range (wavelengths of about 100 μm) since itwould require thicknesses of the semiconductor layers of the order of orgreater than the wavelength, which are absolutely impracticable for thegrowth techniques (MBE, MOCVD) that are generally used. Moreover, sinceinjection devices are involved, the semiconductors used must have apredetermined level of doping to ensure optimal transport properties.This would result into very high losses by absorption since theabsorption coefficient “k” of the free carriers in a doped semiconductoris proportional to the square of the wavelength and thus becomesenormous in the far infrared range (P. Y. Yu and M. Cardona,Fundamentals of Semiconductors, Springer-Verlag, Berlin, 1996).Recently, owing to the development of QCLs with wavelengths greater than15 μm, a new wave-guide based on surface plasmons, has been used (C.Sirtori, et al., Opt. Lett. 23, 1366, 1998; A. Tredicucci et al., Appl.Phys. Lett. 76, 2164, 2000). Surface plasmons are optical modes that areconfined at the interface between two materials with dielectricconstants of opposite sign such as, for example, a metal and asemiconductor. They are TM-polarized (and are therefore very suitablefor QC lasers which emit TM-polarized light) and have an electric-fieldprofile with the maximum at the interface and an exponential decay onboth sides in the direction perpendicular to the surface. If ε₁ is thedielectric constant of the metal and ε₂ is that of the semiconductor,the penetration of the surface plasmon into the two materials is givenby: $\begin{matrix}{\delta_{1,2} = {\frac{\lambda}{2}{{{Re}\lbrack {ɛ_{1,2}\sqrt{\frac{- 1}{ɛ_{1} + ɛ_{2}}}} }^{- 1}}}} & (1)\end{matrix}$

The penetration into the metal layer will thus be less the more negativeis its dielectric constant [Re(ε)=n²−k²]. This aspect is importantbecause the metal is notably absorbent (k>>1) and too pronounced apenetration of the optical mode would cause unacceptable losses. Thisexplains why wave-guides based on surface plasmons are effective onlyfor lasers of sufficiently long wavelength (λ>15 μm) , in which thedielectric constants of the metals become ever more negative (k²>>n²).

The order of magnitude of the loss in surface plasmon wave-guides formedin QC lasers of longer wavelength is about one hundred cm⁻¹ (A.Tredicucci et al., Appl. Phys. Lett. 76, 2164, 2000; R. Colombelli etal., Appl. Phys. Lett. 78, 2620, 2001). Moreover, from the formula givenabove, since the dielectric constant of the semiconductor is relativelysmall and almost exactly real, it also seems clear that the penetrationinto the semiconductor is approximately inversely proportional to thatinto the metal (and in the far infrared range may thus also become veryconsiderable). These characteristics mean that a surface plasmonwave-guide of the type used up to now is also not usable with successfor a THz laser.

The object of the present invention is to provide a laser device whichcan overcome the above-mentioned problems and which is therefore capableof operating effectively at 1-10 THz frequencies.

According to the invention, this object is achieved by a laser devicehaving the characteristics defined in Claim 1.

Preferred embodiments are described in the dependent claims.

In a laser device constructed in this manner, the wave-guide comprises ahighly doped semiconductor layer (preferably with a concentration ofcarriers of the order of 10¹⁸ cm⁻³) which has a negative dielectricconstant but with a modulus suitably matched to the thickness of thelayer (preferably of a few hundred nm) and to the dielectric constant ofthe surrounding material. This permits the formation of a particularoptical mode which is strongly confined to dimensions even less than thewavelength in the material and at the same time with very lowattenuation factors of the order of 10 cm⁻¹.

An advantage of this solution also consists of the fact that is possibleto use this doped layer for the production of an electronic contact withthe active region of the laser, thus making it feasible to use non-dopedsubstrates which are much less absorbent than the doped substratesgenerally used in QCLs.

The use of this wave-guide of novel design permits optical losses in thereal device of barely 17 cm⁻¹ with a factor of confinement of theradiation in the active region of 0.46 at a wavelength of 70 microns.These extremely favourable characteristics enable the laser effect to beachieved even with active materials in which the population inversion isminimal and the gain limited, as is the case in the QC structuresdesigned up to now, in this region of the electromagnetic spectrum.

Further advantages and characteristics will become clear from thefollowing detailed description which is given with reference to theappended drawings, in which:

FIG. 1 is a schematic, perspective view of an embodiment of the presentinvention in a Fabry-Perot laser device with a ridge cavity. Theinsulating layer can be removed, limiting the metallization of the uppercontact to the top of the strip. This is feasible for strip widths >100microns;

FIG. 2 is a graph showing the calculated profile of the fundamental TMoptical mode confined by the presence of an 800 nm thick layer of GaAsn-doped at 5×10¹⁸ cm⁻³ within nominally non-doped GaAs. The wavelengthof the radiation is 70 microns;

FIG. 3 is a graph which shows the calculated profile of the fundamentalTM optical mode within the finished device according to the embodimentof FIG. 1. The upper metal contact has been simulated, taking intoconsideration a thickness of 300 nm and the values obtained in theliterature for gold at the operating wavelength of 70 microns. Theattenuation factor of the mode is about 17 cm⁻¹ with a factor ofconfinement to the active region (indicated by the area in grey) of0.46;

FIG. 4 is a graph which shows the emission spectrum from a 1.2 mm longand 150 μm wide facet of the embodiment of the Fabry-Perot device ofFIG. 1, as a function of the pulsed current applied. Temperature about 8K; and

FIG. 5 is a graph which shows power output as a function of currentapplied for the device of FIG. 1, at various temperatures. The laseremission threshold is about 450 A/cm². Maximum operating temperature 45K.

With reference to FIG. 1, a semiconductor laser 10 comprises an activeregion 12 which can produce a stimulated emission of radiation with acentral wavelength in the far infrared region, in response to a pumpingenergy applied thereto.

The active region 12 is delimited on its lower side by a thin wave-guidelayer 16 which is interposed between the active region 12 and asubstrate 18. The guide layer 16 thus forms an interface 16 a with theactive region 12 and an interface 16 b with the substrate 18.

The guide layer 16 is formed by a semiconductor with high doping,preferably with a concentration of majority carriers of the order of10¹⁸ cm⁻³.

The thin, highly doped semiconductor layer 16 is in conditions such thatthe plasma frequency of the electron gas lies in the middle infraredrange and the real part of the dielectric constant of the layertherefore becomes negative at frequencies in the THz range, whilstremaining (in modulus) of a magnitude more or less comparable with thatof a non-doped semiconductor. As can be seen in FIG. 2, in thesecircumstances, a thin layer having the characteristics of the layer 16and inserted in a normal semiconductor structure permits the formationof a TM mode that is strongly confined close to the layer. Thecalculated intensity profile for a mode of this type at the wavelengthof 70 μm in a non-doped GaAs sample with an 800 nm layer of GaAs n-dopedat 5×10¹⁸ cm⁻³ within it is shown in this drawing. It will be noted thatthe radiation is confined to a total thickness of barely one tenth of amicron, with a pronounced maximum around the doped layer but with verylow intensity within it.

The physical origin of this mode can be understood qualitatively in thefollowing manner. The negative dielectric constant ε₁ of the doped layerinvolves that its interface with the normal semiconductor can supportsurface plasmon modes. The fact that it is only weakly negative,however, entails a considerable penetration into the doped layer which,together with its minimal thickness, results in a coupling of thesurface plasmons of the two interfaces to form the new mode shown inFIG. 2. The spatial extent of this mode in the surrounding semiconductoris controlled by the magnitude of the dielectric constant of the dopedlayer (which can be modified by varying the degree of doping). Asalready pointed out with reference to equation (1), this extent is infact, as a first approximation, directly proportional to the root of−(ε₁+ε₂) and a negative but minimal Re(ε₁) thus results in the closerconfinement of the radiation. However, this does not increase the lossesof the mode in an unacceptable manner, as in the case of a simplesurface plasmon, given the small thickness of the doped layer relativeto its coefficient of absorption. The maximum confinement is achieved,more precisely, when Re(ε₁+ε₂) becomes of the order of Im(ε₁+ε₂), thatis, with dopings of the order of 10¹⁸ cm⁻³ for GaAs. In this embodiment,the best ratio between mode amplitude and losses is obtained with theuse of an n-doping of 2×10¹⁸ cm⁻³ and a thickness of 800 nm, whichcorrespond to a coefficient of absorption of the mode of barely 7 cm⁻¹whilst keeping the confinement at about twenty microns.

With further reference to FIG. 1, the guide layer 16 advantageouslyserves as a base for the electrical contacting of the active region 12by means of a contact 20 disposed directly on the layer 16. It is thuspossible to use non-doped substrates which are much more transparent inthe far infrared than those that are conventionally used. In thisembodiment of the laser device, it has therefore been arranged to growthe doped layer 16 directly on the non-doped GaAs substrate 18 and thento grow the active region 12 described hereinafter (with a thickness ofabout 11 microns). At this point the need to provide another electricalcontact 22, this time on top of the active region 12, requires thedeposition of a metal layer.

In a preferred embodiment of the laser device, this contact 22 isdisposed directly on the active region 12 so as to form an interface 22a therewith. Outside the interface 22 a, the contact 22 is separatedfrom the active region 12 solely by a 200 nm layer GaAs layer 24 dopedat 5×10¹⁸ cm⁻³. This configuration permits a good conductivity of thecontact. A layer 25 of insulating material (for example, SiO₂ or Si₃N₄)may also be deposited beforehand at the sides of the strip (or ridge,which will be discussed in greater detail below) if the lateraldimensions thereof (for example <100 μm) necessitate, for the connection(bonding), an extension of the metallization of the contact 22 wellbeyond the width of the above-mentioned ridge (these are not per se theonly possible solutions and other geometrical arrangements for thedeposition of the upper contact may be provided without, however,departing from the spirit of the invention). The particular consequenceof this choice is that a further surface plasmon bound to the interface22 a with the metal of the contact 22 is mixed with the mode of thewave-guide according to the present invention, resulting in the mode ofthe finished device shown in FIG. 3. The increase in losses is due to alarge extent to the doping, which although low is necessary in theactive region 12, and only to a minimal extent to the presence of theupper metal contact 22. The value of about 17 cm⁻¹ is in any case verylow for a wavelength of 70 microns. The confinement factor Γ of the modein the active region is 0.46. These values indicate the need to achievea gain of at least 30-40 cm⁻¹ in the active region 12.

In one embodiment of the laser device according to the presentinvention, the active region 12 is based on the use ofGaAs/Al_(0.15)Ga_(0.85)As superlattices. Naturally, the invention is notlimited to this particular type of active region of the laser since theinvention is applicable to TM-polarized THz emitters in general. In thisembodiment, the population inversion is achieved by electron injectionbetween the states at the edge of the first energy minigap in theabove-mentioned superlattices. In particular, superlattices ofnon-uniform period or “chirped” are used, enabling well-delocalizedminibands to be maintained even in the presence of the electric fieldthat is necessary for the operation of the device (A. Tredicucci et al.,Appl. Phys. Lett. 73, 2101, 1998 and F. Capasso et al., U.S. Pat. No.6,055,254). The active regions of QC lasers of longer wavelength arebased on this approach (A. Tredicucci et al., Appl. Phys. Lett. 76,2164, 2000, R. Colombelli et al., Appl. Phys. Lett. 78, 2620, 2001) andtheir use at energies less than that of the optical phonon (that is, atTHz frequencies) has recently been discussed (Köhler et al., Appl. Phys.Lett. 79, 3920, 2001). In this latter publication, a particular designof the active material capable of leading to gains of the order of 30cm⁻¹, which are thus compatible with the wave-guide configurationaccording to the invention, was proposed. A series of “chirped”superlattices nominally identical to those described in Köhler et al.,Appl. Phys. Lett. 79, 3920, 2001 and spaced apart by suitable layersthat are designed, according to the usual layout of QCLs, to extract theelectrons from the first miniband of one superlattice and to inject theminto the second miniband of that of the subsequent period, has thereforebeen grown in the region intended for the active material of the laserdevice. In order to cover the required 11 microns, a total of 104 SLunits/injector were required. The complete structure of the sampleproduced, which was capable of emitting at a central wavelength of λ=69μm, is given in detail in Table 1 below. This structure has an overallthickness of 11.9643 μm plus the thickness of the substrate. TABLE 1TYPE COMPOSITION DOPING THICKNESS n⁺⁺ GaAs >5 × 10¹⁸ cm⁻³  200 Å Ninjector   4 × 10¹⁶ cm⁻³ 547 Å Repeated  104× non-doped active zone 502Å N injector   4 × 10¹⁶ cm⁻³  547 Å n⁺⁺ GaAs   2 × 10¹⁸ cm⁻³ 8000 Ånon-doped semi-insulating GaAs (substrate)

The structures of the injector and of the active zone which appear inTable 1 are respectively given in Tables 2 and 3 below. TABLE 2 TYPECOMPOSITION DOPING THICKNESS I GaAs 103 Å I Al_(0.15)Ga_(0.85)As  29 Å NGaAs 4 × 10¹⁶ cm⁻³ 102 Å I Al_(0.15)Ga_(0.85)As  30 Å I GaAs 108 Å IAl_(0.15)Ga_(0.85)As  33 Å I GaAs  99 Å I Al_(0.15)Ga_(0.85)As  43 Å

TABLE 3 TYPE COMPOSITION THICKNESS I GaAs 188 Å I Al_(0.15)Ga_(0.85)As 8 Å i GaAs 158 Å i Al_(0.15)Ga_(0.85)As  6 Å I GaAs 117 Å IAl_(0.15)Ga_(0.85)As  25 Å

The sample was then made into strips (ridges) about 150 microns wide bywet etching to expose the layer with high doping. A metallization stepwas then performed by thermal evaporation of Au/Ge to form the contactsseparately on the two zones of high doping, above and beneath the activeregion, as shown in FIG. 1. Given the dimensions of the device, it wasarranged to perform wive connection (bonding) directly on the contacts,both on top of the ridges and at the side. For thinner ridges it wouldbe necessary to use an insulating layer in order to have a metal surfacelarge enough for the bonding (see FIG. 1). The strips were then definedinto lasers about 1.2 mm long by cleavage along crystalline planesperpendicular to the strips. This left two facets at the ends of eachstrip which act as mirrors to delimit the laser cavity. The devices werethen welded onto copper bars with an In/Ag paste and mounted in acryostat with a continuous flow of helium for the measurements.Naturally, the selection of the geometry and of the characteristics ofthe resonator is not directly connected with the type of wave-guide usedand other configurations (cylindrical cavities, distributed-feedbackresonators, facets with dielectric coating etc.) may be produced withoutaltering the spirit of the invention.

FIG. 4 shows the emission spectrum front a facet, measured at 8 K with aFourier transform interferometer and an Si bolometric detector, as afunction of the supply current of the device. Trains of 750 pulses(duration 200 ns, period 2 μs) were used, repeated at a frequency of 333Hz. This was done in order to have a frequency comparable with theresponse frequency of the bolometer but without heating the sample toomuch. An emission peak at about 18 meV can be observed, well matchedwith the separation energy between the first two minibands of thesuperlattice. The intensity of the signal increases rapidly as thecurrent increases, with a progressive narrowing of the line width up toa current of about 880 mA which identifies the laser threshold. Abovethis, the power increases by several orders of magnitude to a maximumvalue of a few mW and the emission is concentrated in a single mode ofthe cavity with a width of less than one tenth of a cm−1 (the resolutionof the spectrometer used).

The curve of power output as a function of current is given in FIG. 5,for various temperatures. The threshold behaviour typical of laseremission is shown well, with a maximum operating temperature of about 45K.

The performance of the above-described embodiment of the device is stillquite limited but it is stressed that this is purely a firstexperimental construction. In fact there are various possible variationswhich will enable considerable future improvements to be achieved. Forexample, it is expected that some simple solutions such as a reductionin the lateral dimensions of the device, the use of longer ridges, andthe coating of the facets to increase their reflectivity will lead todrastic improvements in terms of power, maximum temperature, andcapacity for continuous operation. The geometry of the wave-guide may inturn be further improved, for example, with the use of thicker activeregions or with a different arrangement of the upper contact. However,the behaviour of the device of the invention as implemented in theabove-described embodiment is excellent per se, with very low losses andlarge confinement factors. Its applicability at different frequenciesand with different active regions throughout the 1-10 THz range isensured.

1. A semiconductor laser comprising: an active region (12) which, inresponse to a pumping energy applied thereto, can produce a stimulatedemission of radiation with a central wavelength (λ) in the far infraredregion, and at least one confinement region (16, 18, 22) suitable forconfining the radiation in the active region (12) and comprising atleast one interface (16 a, 16 b, 22 a) between adjacent layers that iscapable of supporting surface plasmon modes generated by an interactionof the interface with the radiation, characterized in that the at leastone confinement region (16, 18, 22) comprises a wave-guide layer (16)which is delimited on opposite sides by a first interface and by asecond interface (16 a, 16 b), the guide layer (16) being doped in amanner such that the first and second interfaces (16 a, 16 b) arecapable of supporting the plasmon modes, respectively, and the guidelayer (16) being of a thickness (d) such as to bring about theaccumulation of the plasmon modes in proximity to the first and secondinterfaces (16 a, 16 b), outside the layer (16), and substantially asuppression of the plasmon modes, inside the layer.
 2. A laser accordingto claim 1 in which the plasmon modes of the first and second interfaces(16 a, 16 b) are mutually coupled.
 3. A laser according to claim 2 inwhich the wave-guide layer (16) has a dielectric constant (ε₁) with anegative real part and is interposed between regions (12, 18) having adielectric constant (ε₂) with a positive real part but with a modulesubstantially of the same order as the dielectric constant (ε₁) of theguide layer.
 4. A laser according to claim 3 in which the real part ofthe sum of the dielectric constants of the guide layer (16) and of theregions (12, 18) between which the layer is interposed is substantiallyof the order of the imaginary part of the sum.
 5. A laser according toclaim 1 in which the active region (12) comprises a quantum-cascadeactive region.
 6. A laser according to claim 5 in which the activeregion comprises a structure with GaAs/Al_(0.15)Ga_(0.85)Assuperlattices of non-uniform period.
 7. A laser according to claim 1 inwhich the guide layer (16) is interposed between the active region (12)and a substrate region (18).
 8. A laser according to claim 7 in whichthe wave-guide layer (16) is in contact with the active region (12). 9.A laser according to claim 1, further comprising a first electricalcontact region (20) disposed directly on the guide layer (16).
 10. Alaser according to claim 1, further comprising a second electricalcontact region (22) disposed directly on the active region (12).
 11. Alaser according to claim 1, characterized in that it produces astimulated emission of radiation with a frequency of between 1 and 10THz.
 12. A laser according to claim 1 in which the thickness (d) of thewave-guide layer (16) is of the order of 100 nm.
 13. A laser accordingto claim 1 in which the wave-guide layer (16) is formed by an n-typesemiconductor in which the concentration of electrons is of the order of10 ¹⁸ cm⁻³.